1. Field of the Invention
The present invention relates to a light frequency synthesizer which is capable of stably generating CW (continuous wave) light at a freely selected light frequency.
2. Prior Art
Recently, light frequency synthesizers which generate CW light at a pre-specified light frequency have been developed. An example of this type of light frequency synthesizer is shown in FIG. 5. In FIG. 5, reference numeral 32 indicates an input terminal to which a reference electric signal is supplied, and reference numeral 27 indicates a comparison circuit which compares a reference electric signal inputted from the input terminal 82 with a feedback signal described hereinbelow, and which generates a difference signal indicating the difference in the two signals. Reference numeral 28 indicates a light frequency control circuit, which generates a light frequency control signal in accordance with the difference signal supplied from the comparison circuit 27.
Reference numeral 29 indicates an E/O (Electrical signal to Optical signal) conversion circuit; this generates CW light at a light frequency in correspondence with the light frequency control signal supplied from light frequency control circuit 28. Reference numeral 30 indicates a directional coupler, which conducts the CW light outputted from E/O conversion circuit 29 to optical output terminal 33, and which supplies a portion of the CW light to light frequency discriminating 31. Light frequency discriminating 31 discriminates the light frequencies of the CW light outputted from E/O conversion circuit 29, generates an electrical signal indicating this light frequency, and supplies this electrical signal to comparison circuit 27 as the feedback signal described above.
In this manner, by means of the conventional light frequency synthesizer, the feedback control of a closed loop was conducted. That is to say, the CW light outputted from E/O conversion circuit 29 passes through directional coupler 30 and is inputted into light frequency discriminating circuit 31, and is converted to the feedback signal proportional to the light frequency. In comparison circuit 27, the feedback signal and the reference electrical signal are compared, and based on the results of this comparison, the light frequency control circuit 28 controls the light frequency of the E/O conversion circuit 29.
In the conventional light frequency synthesizer described above, in the case in which the closed loop gain is sufficiently large, the degree of stability of the frequency of the CW light is determined by the operational characteristics of the light frequency discriminating circuit 31. Normally, there were a great number of cases in which a Fabry-Perot resonator was employed in the light frequency discriminating circuit 31. However, Fabry-Perot resonators commonly possess a plurality of resonance frequencies, so that it was unclear at which resonance point resonance would occur, and there were flaws in that the absolute value of the light frequency was unclear, it was impossible to control frequencies greater than the resonance interval, and furthermore, such resonators were susceptible to disturbances such as temperature or the like. Accordingly, in structures employing light frequency discriminating circuits 31 having such defects, there was a problem in that it was impossible to generate stable CW light by means of freely selected light frequencies.